The wide bandgap semiconductor materials represented by GaN have many advantages. It is a new type of semiconductor materials which have developed rapidly after the first generation of semiconductor materials represented by silicon (Si) and the second generation of semiconductor materials represented by GaAs. Gallium nitride material has superior physical properties, such as high electron drift velocity, wide bandgap, high thermal conductivity, high pressure resistance, high temperature resistance, radiation resistance, etc. Therefore, it is widely used in high-temperature and high-power devices, optoelectronics and high-frequency microwave devices, and has a large market. Due to the superior electrical performance of AlGaN/GaN HEMT devices and MIS-HEMT devices, they are considered to be the most promising candidates for high-voltage power switches. In recent years, related devices have become an international research hotspot.
In the manufacture process of the semiconductor devices such as GaN, the manufacture of ohmic electrodes is a critical step. To manufacture high-performance GaAs and GaN devices, good ohmic contact is particularly important. If the ohmic contact resistance is too large, the source and drain power dissipation will increase, and the output power and efficiency of the device will be affected. The ohmic contact resistance is an important indicator for judging the ohmic contact quality, and an accurate test of the sheet resistance of the ohmic contact area is the key to calculating the ohmic contact resistance. Therefore, the selection of a suitable method for accurately testing the sheet resistance of the ohmic contact area plays a very important role in the development and evaluation of the device.
Currently, the most common method for measuring ohmic contact resistance is the rectangular transmission line model (TLM) method and the circular transmission line model (CTLM) method.
The rectangular transmission line model (TLM) method is mainly to design a set of rectangular electrode testing patterns with unequal spacing, perform experimental tests, and combine the mathematical calculation, thereby to obtain a value of an active area sheet resistance, then to approximate the active area sheet resistance value Rsh as an ohmic area sheet resistance value Rshc. The method has the following drawbacks: firstly, since the ohmic contact area is formed by a complicated process including metal deposition, ion implantation or high temperature annealing on the active area, the sheet resistance of the active area and the ohmic contact area are not equal, even very different, and cannot be approximated; in addition, the rectangular transmission line patterns themselves need to be mesa-etched at the time of manufacture, which has an edge leakage effect, so the use of this model will make the final measurement result inaccurate.
The circular transmission line model (CTLM) method includes a circular dot transmission line model (CDTLM) method and a circular ring transmission line model (CRTLM) method. The CDTLM method is mainly to design multiple sets of circular dot electrode testing patterns, perform experimental tests and simple curve fitting, thereby to obtain the values of the active area resistance and the ohmic area sheet resistance. Compared with the rectangular transmission line model (TLM) method, the accuracy of this method is improved, but the required testing patterns generally need more than 5 sets, so the test sample has a large area. The CRTLM method is mainly to design three sets of circular ring testing patterns, perform experimental tests and complicated mathematical calculations, thereby to obtain a value of ohmic area sheet resistance. This method has the following drawbacks: first, the CTLM method ignores the sheet resistance of the metal layer during measurement, which will cause a certain calculation error; secondly, the formula for calculating the sheet resistance of the ohmic area is complicated, and the computer software-assisted simulation is needed to solve the Bessel function, which is computationally intensive, resulting in a longer total time for testing and computational analysis.
With the further development of semiconductor devices, the original measurement method for the sheet resistance of the ohmic contact area is more and more difficult to adapt to the requirements of high-performance semiconductor device development. Therefore, it is especially important to correct the original ohmic contact resistance measurement method.